.TH std::ranges::uninitialized_move_n,std::ranges::uninitialized_move_n_result 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::uninitialized_move_n,std::ranges::uninitialized_move_n_result \- std::ranges::uninitialized_move_n,std::ranges::uninitialized_move_n_result

.SH Synopsis
   Defined in header <memory>
   Call signature
   template< std::input_iterator I, no-throw-forward-iterator O,

             no-throw-sentinel-for<O> S >
   requires std::constructible_from<std::iter_value_t<O>,
            std::iter_rvalue_reference_t<I>>                          \fB(1)\fP \fI(since C++20)\fP
   uninitialized_move_n_result<I, O>

   uninitialized_move_n( I ifirst, std::iter_difference_t<I> n, O
   ofirst, S olast );
.SH Helper types
   template< class I, class O >                                       \fB(2)\fP \fI(since C++20)\fP
   using uninitialized_move_n_result = ranges::in_out_result<I, O>;

   Moves N elements from the input range beginning at ifirst to the uninitialized
   storage designated by the range [ofirst, olast), where N is min(n,
   ranges::distance(ofirst, olast)).

   The effect is equivalent to:

 for (; n-- > 0 && ofirst != olast; ++ifirst, ++ofirst)
     ::new (static_cast<void*>(std::addressof(*first)))
         std::remove_reference_t<std::iter_reference_t<O>>(ranges::iter_move(ifirst));

   If an exception is thrown during the initialization then the objects that already
   constructed in [ofirst, olast) are destroyed in an unspecified order. Also, the
   objects in the input range beginning at ifirst, that were already moved, are left in
   a valid but unspecified state.

   The function-like entities described on this page are niebloids, that is:

     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.

   In practice, they may be implemented as function objects, or with special compiler
   extensions.

.SH Parameters

   ifirst        - the beginning of the input range of elements to move from
   ofirst, olast - iterator-sentinel pair denoting the output range of elements to
                   initialize
   n             - the number of elements to move

.SH Return value

   {ifirst + N, ofirst + N}.

.SH Complexity

   Linear in N.

.SH Exceptions

   The exception thrown on construction of the elements in the destination range, if
   any.

.SH Notes

   An implementation may improve the efficiency of the ranges::uninitialized_move_n,
   e.g. by using ranges::copy_n, if the value type of the output range is TrivialType.

.SH Possible implementation

   struct uninitialized_move_n_fn
   {
       template<std::input_iterator I, no-throw-forward-iterator O,
                no-throw-sentinel-for<O> S>
       requires std::constructible_from<std::iter_value_t<O>,
                std::iter_rvalue_reference_t<I>>
       ranges::uninitialized_move_n_result<I, O>
       operator()(I ifirst, std::iter_difference_t<I> n, O ofirst, S olast) const
       {
           O current{ofirst};
           try
           {
               for (; n-- > 0 && current != olast; ++ifirst, ++current)
                   ::new (const_cast<void*>(static_cast<const volatile void*>
                       (std::addressof(*current)))) std::remove_reference_t<
                           std::iter_reference_t<O>>(ranges::iter_move(ifirst));
               return {std::move(ifirst), std::move(current)};
           }
           catch (...) // rollback: destroy constructed elements
           {
               for (; ofirst != current; ++ofirst)
                   ranges::destroy_at(std::addressof(*ofirst));
               throw;
           }
       }
   };

   inline constexpr uninitialized_move_n_fn uninitialized_move_n{};

.SH Example


// Run this code

 #include <cstdlib>
 #include <iomanip>
 #include <iostream>
 #include <memory>
 #include <string>

 void print(auto rem, auto first, auto last)
 {
     for (std::cout << rem; first != last; ++first)
         std::cout << std::quoted(*first) << ' ';
     std::cout << '\\n';
 }

 int main()
 {
     std::string in[]{ "No", "Diagnostic", "Required", };
     print("initially, in: ", std::begin(in), std::end(in));

     if (
         constexpr auto sz = std::size(in);
         void* out = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz)
     )
     {
         try
         {
             auto first{static_cast<std::string*>(out)};
             auto last{first + sz};
             std::ranges::uninitialized_move_n(std::begin(in), sz, first, last);

             print("after move, in: ", std::begin(in), std::end(in));
             print("after move, out: ", first, last);

             std::ranges::destroy(first, last);
         }
         catch (...)
         {
             std::cout << "Exception!\\n";
         }
         std::free(out);
     }
 }

.SH Possible output:

 initially, in: "No" "Diagnostic" "Required"
 after move, in: "" "" ""
 after move, out: "No" "Diagnostic" "Required"

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to              Behavior as published              Correct behavior
   LWG 3870 C++20      this algorithm might create objects on a const  kept disallowed
                       storage

.SH See also

   ranges::uninitialized_move moves a range of objects to an uninitialized area of
   (C++20)                    memory
                              (niebloid)
   uninitialized_move_n       moves a number of objects to an uninitialized area of
   \fI(C++17)\fP                    memory
                              \fI(function template)\fP
